FIG. 1 shows a typical optical lithographic fabrication system 200, for delineating features in a wafer (substrate) 100, such as a semiconductor wafer. More specifically, optical radiation from an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical condensing lens (or condensing lens system) 104, a mask or reticle 103, and an optical imaging lens (or imaging lens system) 102. The optical radiation emanating from the reticle 103 focused by the lens 102 on a photoresist layer 101 located on the top major surface of the wafer 100 itself or, more usually, on various layers on the top surface of the wafer 100. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a developing process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to (printed on) the photoresist. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the substrate or of layer(s) of material(s) (not shown) located between the top surface of the wafer and the bottom surface of the photoresist layer, or of both the substrate and the layer(s). Portions of the substrate or of the layer(s) of material thus are removed from the top surface of the wafer 100 underlying areas where the photoresist was removed by the developing process but not underlying areas where the photoresist remains. Alternatively, ions can be spatially selectively introduced into the wafer 100 or into layer(s) of material(s) overlying the wafer. Thus, the pattern of the mask 103 is transferred to the wafer 100 or to layer(s) of material(s) overlying the wafer 100, as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable to have as many devices, such as transistors, per wafer. Hence it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an isolated aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and another. Thus, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, there must exist a feature having a width equal to C located on the mask (reticle) 103. According to geometric optics, if this feature of width C is a simple (localized) aperture in a (wide-field) opaque layer, then the ratio W/C=m, where m=L2/L 1, i.e., the image distance divided by the object distance, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become fuzzy and lose their sharpness; and hence the so-called resolution of the mask features when focused on the photoresist layer 101 deteriorates.
In a paper entitled "New Phase Shifting Mask with Self-Aligned Phase Shifters for a Quarter Micron Lithography" published in International Electron Device Meeting (IEDM) Technical Digest, pp. 57-60 (3.3.1-3.3.4) (December, 1989), A. Nitayama et al. taught the use of masks having such features as isolated apertures transparent phase-shifting portions to achieve improved resolution--i.e., improved sharpness of the image of the mask features when focused on the photoresist layer 101. More specifically, these masks comprised suitably patterned transparent optical phase-shifting layers, i.e., layers having edges located at predetermined distances from the edges of the opaque portions of the mask. Each of these phase-shifting layers had a thickness t equal to .lambda./2(n-1), where .lambda. is the wavelength of the optical radiation from the source 106 (FIG. 1) and n is the refractive index of the phase-shifting layers. Thus these layers introduced phase shifts (delays) of .pi. radian (or some other odd integral multiple of .pi. radian) in the optical radiation. In general, the phase shift .phi. that is introduced by one layer relative to another depends upon the respective thicknesses t.sub.1 and t.sub.2 and upon the respective refractive indices n.sub.1 and n.sub.2 of these layers: .phi.=2.pi.(n.sub.2 t.sub.2 -n.sub.1 t.sub.1)/.lambda.; whereby in a case where .phi.=.pi., n.sub.1 =1, and t.sub.1 =t.sub.2 =t, it follows that t=.lambda./2(n.sub.2 -1). By virtue of diffraction principles, the presence of these phase-shifting layers in the masks assertedly produces the desired improved resolution. Such masks are called "phase-shifting" masks.
The mask taught by A. Nitayama et al., for printing an isolated aperture feature in a resist layer located on the wafer 100, has an isolated aperture of width, say A, in an opaque layer with a phase-shifting layer located on the opaque layer. This phase-shifting layer overhangs (extends beyond) the peripheral edges of the opaque layer by a distance equal to B. Thus the phase-shifting layer extends into the aperture in the opaque layer by the distance B all around the periphery of the aperture. That is, a phase-shifting rim of width B is formed. In this way, the width C of the remaining, non-phase shifting portion of the aperture feature is equal to A-2B. Nitayama et al. further taught that this width C=A-2B is to be made equal to W/m, where W and m are defined as above. That is to say, Nitayama et al. further taught that C=W/m; Cm/W=1. Typically, m=1/5, so that Nitayama et al. thus further taught that typically C=5W. The teachings of Nitayama et al. do not result in as large a resolution improvement, if any, as is desired from the use of the phase-shifting mask. Therefore, it would be desirable to have a new teaching for phase-shifting masks that yields a larger resolution improvement for isolated aperture features.